This invention relates to the fabrication and access of magnetic devices used in, for example, magnetic memory cells in a magnetic random access memory (xe2x80x9cMRAMxe2x80x9d).
Magnetic Random Access Memory (xe2x80x9cMRAMxe2x80x9d) arrays of the type disclosed in the two above-incorporated U.S. Patents, and depicted in FIGS. 1a-b herein, include an array of magnetic memory cells (e.g., cell 9) positioned at the intersections of wordlines 1, 2, 3 and bitlines 4, 5, 6. Each cell includes a magnetically changeable or free region 24, and a proximate magnetic reference region 20, arranged into a magnetic tunnel junction (xe2x80x9cMTJxe2x80x9d) device 8. The principle underlying storage of data in such cells is the ability to change the relative orientation of the magnetization of the free and reference regions by changing the direction of magnetization along the easy axis (xe2x80x9cEAxe2x80x9d) of the free region, and the ability to thereafter read this relative orientation difference.
More particularly, an MRAM cell is written by reversing the free region magnetization using applied bi-directional electrical and resultant magnetic stimuli via its respective bitline and wordline, and is later read by measuring the resultant tunneling resistance between the bitline and wordline, which assumes one of two values depending on the relative orientation of the magnetization of the free region with respect to the reference region. (The term reference region is used broadly herein to denote any type of region which, in cooperation with the free or changeable region, results in a detectable state of the device as a whole.) If the free region is modeled as a simple elemental magnet having a direction of magnetization which is free to rotate but with a strong preference for aligning in either direction along its easy axis (+EA or xe2x88x92EA), and if the reference region is a similar elemental magnet but having a direction of magnetization fixed in the +EA direction, then two states (and therefore the two possible tunneling resistance values) are defined for the cell: aligned (+EA/+EA) and anti-aligned (xe2x88x92EA/+EA).
An ideal hysteresis loop characterizing the tunnel junction resistance with respect to the applied EA field is shown in FIG. 2. The resistance of the tunnel junction can assume one of two distinct values with no applied stimulus in region 50, i.e., there is a lack of sensitivity of resistance to applied field below the easy axis flipping field strength xc2x1Hc in region 50. If the applied easy axis field exceeds xc2x1Hc, then the cell is coerced into its respective high resistance (anti-aligned magnetization of the free region with respect to the reference region) or low resistance (aligned magnetization of the free region with respect to the reference region) state.
Even if the magnetization pattern of the two regions forming the tunnel junction is simple, reversing the direction of magnetization in the free region during writing can actually affect one or both regions in unexpected ways. For example, the reversal of the free region during writing can result in the inclusion of a magnetic vortex or complex magnetic domain walls, pinned by a defect or by edge roughness. Because the junction resistance depends on the dot product mfreemreference averaged over the junction area, inclusion of such complex micromagnetic structures in the magnetization pattern can substantially corrupt the measured tunnel junction resistance during reading.
For example, shown in FIG. 3 is the magnetization pattern in a free magnetic region 59 formed symmetrically about its easy axis EA in which a complicated wall structure is clearly evident between otherwise acceptable magnetization pattern regions. This overall magnetization pattern was attained from a nominally uniformly magnetized sample (both top and bottom layers originally pointing to the right), for which the easy axis bias was swept from +700 Oe to xe2x88x92700 Oe and back to +700 Oe. The reversal of magnetization evolved to a complicated structure as the field was swept from +700 Oe down to about xe2x88x92280 Oe. FIG. 4 is a hysteresis loop depicting the net direction of magnetization averaged over the device versus applied easy axis field for this corrupt sample. The non-square nature of region 50, resulting in a cell which will not predictably assume either one of its two states upon the removal of the easy axis applied field, is due to the evolution of such complex micromagnetic structures in the cell.
Some improvements in this situation are possible. For example, in the above-incorporated U.S. Patent Application entitled xe2x80x9cINTENTIONAL ASYMMETRY IMPOSED DURING FABRICATION AND/OR ACCESS OF MAGNETIC TUNNEL JUNCTION DEVICES,xe2x80x9d the present inventors have disclosed a technique for avoiding the evolution of some of the undesirable micromagnetic structures in typical MRAM cells. Substantial improvements have been demonstrated, and in the best of cases no wall structures evolve during the cycling of fields used for reversing magnetization. However, as shown in the free region 69 of FIG. 5, for even these improved conditions, there can still be a substantial twist in the magnetization pattern. This twist causes the undesirable, rounded shape in the resistance versus field hysteresis loop shown in FIG. 6.
Though the problems discussed above relate to magnetic tunnel junction devices, similar problems exist in any magnetic device in which a magnetoresistive effect is used as a basis for electrical interaction (e.g., giant magnetoresistive (xe2x80x9cGMRxe2x80x9d) devices). This interaction is broadly referred to herein as magnetoresistive electrical interaction.
The non-ideal behavior in the magnetization reversal process in such devices results in a reduction in the useful parametric window of operation at best, or a total collapse of the square hysteresis loop necessary for storage at worst. What is required, therefore, is an improvement in the electrical performance of a well-behaved magnetoresistive device even if the magnetization patterns in the free region do not uniformly assume a single one of two possible directions of magnetization.
The present inventors have realized that the electrical and magnetic properties of magnetoresistive devices are to some extent separable. The electrical interaction region, for example, a tunneling region, can therefore be modified to allow regions thereof to be more or less electrically conductive, while leaving the overall magnetic structure and evolution patterns of the device virtually unchanged. This modification enables the limiting of the electrical interaction with the device to a preferred portion of the free magnetic region, thereby minimizing or completely eliminating the effects of the above-described, undesirable magnetization patterns in other portions of the free region.
In that regard, the present invention, in one aspect, relates to a magnetoresistive device, having a first magnetic layer formed in conjunction with at least one other structure in said device, such that upon magnetoresistive electrical interaction therewith, said electrical interaction (e.g., electrical tunneling) occurs only through a preferred portion of the first magnetic layer and not any remaining portion thereof. The first magnetic layer may be changeable into one of at least two substantially opposing magnetic states along an axis thereof, and the preferred portion of the first magnetic layer may be centered about a midpoint of the axis. The preferred portion of the first magnetic layer may be less than 50% of the size of the first magnetic layer measured in a first lateral dimension parallel to the axis.
To limit the electrical interaction only to the preferred portion of the first magnetic layer, the at least one other structure in the device may be an electrical interaction region smaller than the first magnetic layer and arranged in a conductive relationship to the preferred portion of the first magnetic layer, thereby effecting electrical interaction only through the preferred portion of the first magnetic layer, and not the remaining portion thereof. The at least one other structure in the device may also comprise an electrically insulating region arranged in an insulating relationship to the remaining portion of the first magnetic layer, but not the preferred portion thereof, thereby effecting electrical interaction only through the preferred portion of the magnetic layer and not the remaining portion thereof.
The magnetoresistive device can be used as a magnetic memory cell in a magnetic memory, or as an access element adapted to access data on a magnetic data storage medium.
In another aspect, the present invention relates to a method for accessing a magnetoresistive device having a first magnetic region changeable into each of two magnetic states. The method includes limiting electrical interaction to only a preferred portion of the first magnetic region, and not any remaining portion thereof. The preferred portion of the first magnetic region comprises a region wherein each of two magnetic states into which the region is changeable can be dependably predicted to be substantially uniform and opposite of one another.
The method may include using an electrical interaction region formed to effect electrical interaction only through the preferred portion of the first magnetic region and not the remaining portion thereof. This may be accomplished by forming the interaction region to be electrically conductive proximate to the preferred portion of the first magnetic region, and using insulation formed to prevent electrical interaction through the remaining portion of the first magnetic region.
In yet another aspect, the present invention relates to a method for forming a magnetoresistive device, including forming an electrical interaction region through which electrical interaction will occur upon access of said device. A first magnetic layer changeable into each of two magnetic states is formed proximate to, and larger than, the interaction region such that upon said access said electrical interaction will occur only through a preferred portion of the first magnetic layer, determined by the resultant position of the interaction region proximate to which the first magnetic layer is formed, and not any remaining portion thereof.
The formation of the interaction region may include decreasing an electrically insulative effect in a given region of an otherwise insulating region thereby forming the interaction region in the given region. Decreasing the electrically insulating effect in the given region may include providing less electrical insulation in the given region.
The formation of the interaction region may also include forming electrical insulation in areas at least partially around the interaction region to prevent electrical interaction in these areas and therefore in the remaining portion of the first magnetic layer. The electrical insulation may be formed by depositing the insulation in these areas at least partially around the interaction region, or by ion-implanting these areas to convert the areas from an otherwise non-insulating material into an insulating material, while isolating the interaction region from the ion-implanting, thereby maintaining an electrically conductive characteristic of the interaction region.
By limiting electrical interaction to only a preferred portion of the free magnetic region, within which the direction and uniformity of the two states of magnetization can be dependably predicted, the resultant resistance response, and therefore the overall electrical interaction response, is improved.